This invention relates to a calibration checking device and related method for setting and calibrating limit switches.
Many complex systems such as nuclear power plants include a large number of motor driven devices such as valves. Typically limit switches are provided on motor driven valves to allow the valves to be remotely opened and closed. One commonly used limit switch is sold under the trade name "Limitorque." Such limit switches have at least one upper limit switch, at least one lower limit switch, and a limit switch mechanism for controlling the upper and lower limit switches in response to movement of an input member such as a gear. Typically, these limit switches are used in situations where separate signals are needed to define the points at which the motor should be deactivated as the gate nears its open and closed positions.
Such limit switches must be removed periodically for servicing, and must occasionally be replaced. In either case, it is important that the reinstalled limit switch provide the desired switch points for the upper and the lower limit switches. This means that the travel between the upper and lower limit switches must be within specification, and the position of the limit switch must correspond to the position of the valve.
For example, when limit switches are removed for cleaning and regreasing, internal gears of the limit switch mechanism can easily be moved from their original setting. If such an error occurs and is not detected before the limit switch is reinstalled, the limit switches will have to be recalibrated after installation onto the valve. This means costly delays in terms of increased down-time and it may involve unnecessary radiation exposure for maintenance personnel. If the miscalibration of the limit switch were undetected entirely, there would be a possibility for valve over-travel and resulting valve damage, because it is the limit switches that control the range of valve travel.
Limit switches have been manually calibrated in the past while removed from valves and the like. However, simple manual calibration is prone to error, since the calibration process is under manual control.
Thus, a need exists for an improved system for calibrating limit switches that is accurate, consistent, and that operates with minimal opportunity for human error. Preferably, such a system should minimize radiation exposure for maintenance personnel, as well as down-time and calibration time.